Vermiculite interlayer water

It is believed that CuO O) " - orients in vermiculite interlayers (2 water layers thick) as shown in Figure 18B (6). Yet, the smectites with more vermiculite-like properties (high tetrahedral charge, high total charge) showed no evidence of this orientation, even in cases where two layers of water were situated between the plates. It is necessary to conclude that Cud O) " or Cu(H20)5 + ions are found in the two-layer hydrates of the smectites, with the orientation shown in Figure 18C. 

Considerable advances in the understanding of the structure of interlayer water in both smectites and vermiculites have been also achieved by means of computational simulations [61]. 

The compression of the interlayer water between the unit layers of the vermiculite clay is probably a consequence of the large attractive force between the negatively charged unit layers and a densely populated layer of sodium ions midway between the unit layers. This large attractive force also keeps the unit layers from swelling beyond a two-layer complex. 

Note Cations listed first in curved brackets for the smectites and vermiculites (Na, Ca, K, and Mg) arc present as exchangeable interlayer ions. All the smectites and vermiculites (and thus interlayer iilite-smectites) have important amounts of interlayer water, the amount of which depends upon the clay and the nature of interlayer cations (cf. Brindley and Brown 1980). As is customary, these waters are left out of the mineral formulae. 

The maximum amount of interlayer water taken up by vermiculite, even when the sample is immersed in liquid water, is two layers for Mg or Ca saturation (Barshad, 1948, 1950 Walker, 1956 van Olphen, 1963). There is one interlayer for Na saturation, and there is some interlayer water present in K -saturated vermiculite, because heating causes a basal spacing decrease to 10.1 A, but the decrease is less than that required for a water layer in vermiculites saturated with the other cations mentioned. 

Walker (1956,1957) and van Olphen (1963) discussed the conditions of temperature and humidity at which partly-hydrated Mg-vermiculites exist in air. Walker interpreted differential thermal analyses (DTA) charts as indicating that interlayer water not associated with the hydration of adsorbed Mg ions is released at a lower temperature than is that around the Mg. Most of both of these types of interlayer water is released by 275°C. Vermiculite held under 63 MPa (10,000 p.s.i.) water vapor pressure shows its first dehydration at 550°C (Roy and Romo, 1957). 

Woessner [56] also considered the effects of proton exchange on the dipolar oscillations arising from oriented water in chabazite and vermiculite. This paper involves a nice application of the density matrix for calculation of the effect of exchange on the FID. Comparisons are made between the proton exchange rate measured for interlayer water and pure water, and it was found that the rate was very dependent on the clay structure and cation involved. Fripiat reviewed the area of proton exchange on acid catalysts in 1976 [59]. The findings of Woessner [56] are not in complete agreement with conclusions reached in this review, which assert that dissociation of water is more pronounced in layered materials. Woessner s results seem to indicate that this is not necessarily the case. 

Skipper NT, Soper AK, McConnell JDC (1991). The structure of interlayer water in vermiculite. J Chem Phys 94 5751-5760... 

G. D. Boss and E. O. Stejskal, Restricted, anisotropic diffusion and anisotropic nuclear spin relaxation of protons in hydrated vermiculite crystals, J. Colloid Interface Sci. 26 271 (1968). S. Olejnik and J. W. White, Thin layers of water in vermiculites and montmorillonites Modification of water diffusion. Nature Phys. Sci. 236 15 (1972). J. Hougardy, J. M. Serratosa, W. Stone, and H. van Olphen, Interlayer water in vermiculite Thermodynamic properties, packing density, nuclear pulse resonance, and infrared absorption, Spec. Disc. Faraday Soc. 1 187 (1970). 

Vermiculite is a hydrous, silicate mineral, which exfoliates greatly when heated sufficiently. The structure of vermiculite consists of 2 tetrahedral sheets for every one octahedral sheet. It has medium shrink-swell capacity with limited expansion. The cation exchange capacity is high in the range of 100-150 meq/100 g. The structure of typical vermiculite contains a central octahedrally-coordinated layer of Mg ions, which lies between two inwardly pointing sheets of silicate tetrahedra. These silicate layers are normally separated by two sheets of interlayer water molecules. Complete removal of water molecules leads to 9.02 A lattice. These layers are electrically neutral and interlayer cations occupy only about one-third of the available sites. The cohesion between the layers is typically weak [10]. 

The foregoing discussion indicated that several phyllosihcate minerals, either naturally or as the result of chemical treatment, have molecular species inserted between the siUcate layers. Water is the most common interlayer sp>ecies in nature, and water is normally found in smectites, vermiculites and hydrated halloysites. The quantity of interlayer water is a function of relative humidity and the type of interlayer cation, in the case of smectites and vermiculites. 

The interlayer water content is very dependent on the nature of the interlayer cation in general, the more highly hydratable the ion, the greater the amount of interlayer water. Thus, Cs-vermiculite wiU contain almost no interlayer water under normal conditions of temperature and humidity, while Mg-vermiculite contains between 4 and 5 H2O per Oio(OH)2 unit of structure. 

In 1934, the first insights into the structure of vermicuUte were obtained by two independent workers using X-ray powder methods. Kazantzev, on the one hand, reported that the unit cell is analogous to that of biotite, but of slightly larger dimensions, with K partly replaced by H and Fe by Mg. The other, Gruner, showed that the structure consists of silicate layers resembling those of mica or talc, with double sheets of water molecules between them. These so-called interlayer water molecules occupy a space very nearly equal to that occupied by a brucite layer in the chlorite structure, with the result that the X-ray diffraction effects obtained from vermiculites and chlorites have certain similarities. 

Figure 3. A system of bonding of the interlayer water in Mg-vermiculite proposed by Bradley and Serratosa [I960]. A 16 x 9.2 A section contains two octahedra of water molecules coordinated about two Mg exchange ions. Open hexagonal nets envelop the filled octahedra. O-H. .. O bonds involving only water are shown as filled lines, with protons indicated along these lines for the upper water layer only. Four water molecules furnish both their protons to 3.0 A hydrogen bonds in the plane, and eight furnish one proton to a long bond and one to a short. Two molecules (H) have no specific second neighbor.

Using infrared spectroscopy, Fripiat et al. [1960] have shown that some interlayer water molecules are trapped in the lattice of a South African vermiculite during contraction by heat treatment and that dehydroxylation starts before the final removal of the interlayer water. Warshaw et al. [1960] examined the X-ray diffraction patterns of a vermiculite from Westtown, Pennsylvania, after heating at various temperatures for periods of 11 to 16 hr, but made no attempt at a structural interpretation of their data. After heating at 120°C, they observed that the 14.3 A basal reflection was displaced to 11.6 A at 270°C, a broad band consisting of peaks at 12.6 and 10.8 A was found at 400° and 475°C, the broad band consisted of peaks at 10.2 and 8.7 A and at 500°C, the broad band was centered on 9.6 A with a slight shoulder at 8.7 A. 

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